Optical waveguide coupler

ABSTRACT

The present invention provides an optical waveguide coupler which relaxes the phase matching condition and facilitates the coupling between an optical waveguide and a resonator.  
     The light in a multimode optical waveguide ( 1 ) is coupled directly to a microsphere resonator ( 2 ) via evanescent light. Since the size of the microsphere resonator ( 2 ) is very small, the efficiency of coupling between the microsphere resonator ( 2 ) and the multimode optical waveguide ( 1 ) is low. Therefore, under the nonresonance condition, the influence of the microsphere resonator ( 2 ) is small, and light propagation through the multimode optical waveguide ( 1 ) is not influenced. However, since the intensity of the light stored in the microsphere resonator ( 2 ) is high under the resonance condition, even if the coupling efficiency is low, the microsphere resonator ( 2 ) outputs light which has approximately the same power as that of the light in the multimode optical waveguide ( 1 ). Therefore, strong coupling condition is always satisfied, and the optical waveguide coupler functions as a filter due to the interference between the light in the multimode optical waveguide ( 1 ) and the light that has passed through the microsphere resonator ( 2 ).

TECHNICAL FIELD

[0001] The present invention relates to an optical waveguide couplerand, more particularly, to a technique for relaxing the phase matchingcondition of a coupler existing in a transmission path.

BACKGROUND ART

[0002] Conventionally, when a resonator is connected to an opticalwaveguide in order to provide a frequency discrimination function, phasematching between the guided mode of the optical waveguide and that ofthe resonator has been considered important for improving the efficiencyof signal transmission between the optical waveguide and the resonator.

DISCLOSURE OF THE INVENTION

[0003] Conventional optical waveguide couplers having a wavelengthdiscrimination function require a special design for realizing the phasematching condition to thereby obtain a sufficiently high degree ofcoupling between a mode in a optical waveguide and that in a coupler.Specifically, the shape of a coupling portion, refraction index ofmaterial, coupling length, etc. must be designed precisely inconsideration of various factors, such as distortion of the guided modesresulting from the coupling.

[0004] A long coupling section has been required to attain sufficientlyhigh transmissibility (see, for example, Japanese Patent ApplicationLaid-Open (kokai) Nos. 07-301716 and 08-248229).

[0005]FIG. 1 is a schematic view of such a conventional opticalwaveguide coupler (optical tapping) (see Japanese Patent ApplicationLaid-Open No. 08-248229). As is apparent from FIG. 1, a long couplingsection 31 is needed in order to satisfy the phase matching condition.

[0006] In view of the foregoing, an object of the present invention isto provide an optical waveguide coupler which relaxes the phase matchingcondition of the above-described conventional technique and facilitatesthe coupling between an optical waveguide and a resonator.

[0007] To achieve the above object, the present invention provides thefollowing optical waveguide couplers.

[0008] [1] An optical waveguide coupler including a resonator which isin contact with a plurality of optical waveguides and has a wavelengthdiscrimination function, characterized in that a micro-cylinderresonator is sandwiched between two optical circuit boards each carryinga plurality of optical waveguides such that guided light propagatingthrough each optical waveguide of one optical circuit board and having awavelength satisfying the resonance condition of the resonatorpropagates via the resonator to the corresponding optical waveguide ofthe other optical circuit board and exits from an output end of thecorresponding optical waveguide, and that, by utilization of a sizeeffect attained through size reduction of the resonator, influence ofthe resonator on signal transmission between the optical waveguide andthe resonator is reduced under a nonresonance condition, and a conditionfor phase matching between a guided mode of the optical waveguide andthat of the resonator is relaxed under a resonance condition.

[0009] [2] In the optical waveguide coupler as described above in [1],the size effect of the resonator is such that a size parameter L/λ fallswithin a range of 10 to 100, where L is a peripheral length of theresonator, and λ is the wavelength of light in vacuum.

[0010] [3] In the optical waveguide coupler as described above in [1] or[2], a micro-resonator is disposed in contact with a multimode opticalwaveguide in order to provide a function of a notch filter which reducesto zero intensity of light transmitted to a specific mode that satisfiesthe resonance condition of the multimode optical waveguide and themicro-resonator.

[0011] The present invention is characterized in that a condition forphase matching is relaxed greatly by use of a micro-resonator serving asa coupler. That is, an optical waveguide from which evanescent wavesleak (e.g., optical fiber having its clad layer removed) is brought intocontact with a very small sphere or cylinder so as to obtain an opticalwaveguide coupler. The relaxation of the phase matching condition occursdue to a small contact point between the very small sphere or cylinderand the optical waveguide. Therefore, the coupling constant of thecoupler is small, and in general, the coupler hardly affects the opticalwaveguide.

[0012] Meanwhile, at a wavelength at which light resonates with theresonator, light intensity within the resonator is high. Therefore, evenwhen the coupling is weak, light having a sufficiently high intensity isintroduced into the optical waveguide, and is caused to interfere withthe guided light. Thus, the coupler exhibits a wavelength discriminationproperty.

[0013] The present invention has the following advantages.

[0014] (1) A resonator to be used is very small; and when the resonatoris brought into contact with an optical waveguide, a coupling lengthbecomes short, so that the phase matching condition can be relaxed.

[0015] (2) Since the density of optical waves within the micro-resonatorbecomes very high under resonance condition, signal transmission can beperformed sufficiently even when the coupling length is short.

[0016] As described above, the problems of the conventional techniquecan be solved by use of a very small resonator.

BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a schematic view of a conventional optical waveguidecoupler (optical tapping).

[0018]FIG. 2 is a sectional view showing an optical waveguide coupleraccording to a first embodiment of the present invention.

[0019]FIG. 3 is a graph showing results of an experiment performed onthe wavelength dependency of light intensity of a certain output modeemployed in the optical waveguide coupler shown in FIG. 2.

[0020]FIG. 4 is a graph showing results of calculation of distributionof propagation coefficients within a multimode optical waveguide forcase 1 in which the size parameter of a micro-resonator is 26 and case 2in which the size parameter is 1000, as well as showing that when thesize parameter is small, the resonator can be coupled to a larger numberof modes within the optical waveguide.

[0021]FIG. 5 is a sectional view showing an optical waveguide coupleraccording to a second embodiment of the present invention.

[0022]FIG. 6 is a perspective view showing an optical waveguide coupleraccording to a third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] Embodiments of the present invention will next be described indetail with reference to the drawings.

[0024]FIG. 2 is a sectional view of an optical waveguide coupleraccording to a first embodiment of the present invention in which amultimode optical waveguide and a micro-resonator are in mutual contact.

[0025] In FIG. 2, reference numeral 1 denotes a multimode opticalwaveguide (glass plate whose refraction index is 1.5); and referencenumeral 2 denotes a microsphere resonator (a glass sphere whoserefraction index is 1.5 and whose diameter is 5 microns) which is incontact with the multimode optical waveguide 1.

[0026] An arrow shows a mode to be considered among an innumerablenumber of modes of the multimode optical waveguide 1; and slanted linesschematically show phase of light; i.e., propagation direction of lightwithin the microsphere resonator 2 and those of light output from themicrosphere resonator 2 to the multimode optical waveguide 1. FIG. 2shows that intensity of output light changes as a result of interferencebetween the two light rays.

[0027] Light in the multimode optical waveguide 1 is coupled directly tothe microsphere resonator 2 via evanescent waves. Since the size of themicrosphere resonator 2 is very small, the efficiency of couplingbetween the microsphere resonator 2 and the multimode optical waveguide1 is low. Therefore, under the nonresonance condition, the influence ofthe microsphere resonator 2 is small, and light propagation through themultimode optical waveguide 1 is not influenced. However, since theintensity of the light stored in the microsphere resonator 2 is highunder the resonance condition, even if the coupling efficiency is low,the microsphere resonator 2 outputs light which has approximately thesame power as that of the light in the multimode optical waveguide 1.Therefore, strong coupling condition is always satisfied, and theoptical coupler functions as a filter by virtue of the interferencebetween the light in the multimode optical waveguide 1 and the lightthat has passed through the microsphere resonator 2. Here, a normalizedpropagation coefficient is defined as a ratio of an in-plane componentof the wavenumber of the light propagating through the multimode opticalwaveguide 1 to a value obtained by dividing the wavenumber of light invacuum by the refraction index of the multimode optical waveguide 1. Inthe present embodiment, an optimal filter action was experimentallyobtained when output to a mode having a normalized propagationcoefficient of 0.77 was observed.

[0028]FIG. 3 shows the results of the experiment. In FIG. 3, thevertical axis represents light intensity, and the horizontal axisrepresents wavelength. The optical waveguide coupler functions as anotch filter which has characteristics such that the intensity of lighttransmitted to the above-described mode in the multimode opticalwaveguide 1 becomes zero at certain wavelengths.

[0029] Moreover, since the size of the microsphere resonator 2 is verysmall, the area of contact between the microsphere resonator 2 and themultimode optical waveguide 1 becomes very small, thereby obviatingstrict holding of a propagation coefficient conservation law (phasematching). Specifically, calculated propagation coefficients of opticalmodes output from the sphere spread in a wide range as indicated bycurve a in FIG. 4, due to a diffraction effect caused by light enteringfrom a very small contact point; and effective coupling is attained ifthe propagation coefficient of the multimode optical waveguide 1 fallswithin the spread range.

[0030] By contrast, when a large resonator is used, as indicated by lineb in FIG. 4, the propagation coefficients of optical modes output fromthe sphere hardly spread. Therefore, in order to attain effectivecoupling, the propagation coefficient of the optical waveguide must bemade precisely coincide with that of the optical mode output from thesphere. For more general description, a size parameter is defined as theratio of the resonator peripheral length to the optical wavelength invacuum. When the size parameter is 100 or less (although a slight changeoccurs due to changes in the light confinement effect of the resonatordepending on refraction index; here, calculation is performed on theassumption that the resonator is formed of a glass sphere whoserefraction index is 1.5), the condition of phase matching with themultimode optical waveguide 1 can be relaxed greatly. However, when thespectral width of a resonance mode approaches the mode interval betweenadjacent resonant modes, a plurality of resonant modes overlap oneanother. In this case, the optical waveguide coupler does not functionas a notch filter. Therefore, the lower limit of the size parameter isdetermined in view of the above.

[0031] The spectral widths of resonance modes and mode intervals areknown to exhibit close agreement between their experimental values andcalculated values; and these values can be obtained from a sizeparameter and refraction index. Through such calculation, the lowerlimit of the size parameter is estimated to be about 10. Thecharacteristic of relaxing the phase matching condition is obtainednaturally by use of a revolving-type micro-resonator, irrespective ofconfiguration of the optical waveguide and materials (refractionindexes, distribution relation, etc.) of the optical waveguide and therevolving-type micro-resonator, although the range of the effective sizeparameter is finely corrected in accordance with the refraction indexesof the optical waveguide and the revolving-type micro-resonator.

[0032] Next, a second embodiment of the present invention will bedescribed.

[0033]FIG. 5 is a sectional view of an optical waveguide coupleraccording to the second embodiment of the present invention, which has amicrosphere in contact with two optical waveguides. A cross section of amicrosphere resonator 11, which is in contact with two opticalwaveguides 12 and 13, is shown. Characteristics of an addition filterare produced between an inlet 14 and an outlet 16; and characteristicsof a removal filter are produced between the inlet 14 and an outlet 15.

[0034] Specifically, the microsphere resonator 11 is formed of a glasssphere (refraction index: 1.5, diameter: 5 microns) and is in contactwith two optical waveguides 12 and 13. As described in relation to thefirst embodiment, the optical waveguide 12 and 13 can assume any form,so long as the optical waveguide 12 and 13 can be coupled to themicrosphere resonator 11 via evanescent waves. In the example shown inFIG. 5, an optical fiber having its core exposed is used. The opticalwaveguide 12 serves as an input waveguide. On the basis of the sameprinciple as that of the first embodiment, strong coupling is alwaysestablished between the microsphere resonator 11 and the input waveguide12 and between the microsphere resonator 11 and the output waveguide 13.Therefore, when observed from the inlet 14 of the input waveguide 12,impedance matching is established at all times. Therefore, atwavelengths that satisfy the resonance condition of the resonator, anoptical signal is output from the outlet 16 of the waveguide 13 almostcompletely. At other wavelengths, an optical signal is output from theoutlet 15 of the input waveguide 12 almost completely. Thus, throughemployment of the above-described configuration, an optical channeladdition/removal filter is realized with ease.

[0035] Next, a third embodiment of the present invention will bedescribed.

[0036]FIG. 6 is a perspective view of an optical waveguide coupleraccording to the third embodiment of the present invention, which has amicro-cylinder resonator disposed between two optical circuit boardscarrying waveguides on their surfaces which serves as a connector havinga wavelength selection function.

[0037] As shown in FIG. 6, two optical circuit boards 22 and 23 carryingchannel-type waveguides on their surfaces are used as waveguides.Further, a very small cylinder is used as a micro-cylinder resonator 21.The micro-cylinder resonator 21 is sandwiched between the opticalcircuit boards 22 and 23 to thereby realize an inter-board opticalconnector having a wavelength filter function by employment of the sameprinciple as that of the second embodiment.

[0038] The present invention is not limited to the above-describedembodiments. Numerous modifications and variations of the presentinvention are possible in light of the spirit of the present invention,and they are not excluded from the scope of the present invention.

[0039] As described in detail above, the present invention can achievethe following effects.

[0040] The present inventors found that the phase matching condition,which has been considered to become important when an optical waveguideis coupled with other optical circuit, is relaxed greatly when theoptical waveguide is coupled with a micro-resonator; i.e., the couplingefficiency can be low, because of high Q value of the resonator. Thiscontributes to simplification of design conditions of an optical couplerhaving a wavelength discrimination function.

[0041] When the phase matching condition is relaxed by the presentinvention, a resonator can be coupled to an optical waveguide,regardless of whether or not their guided modes coincide.

[0042] In other words, the present invention provides the followingeffects.

[0043] (1) A resonator to be used is very small; and when the resonatoris brought into contact with an optical waveguide, a coupling lengthbecomes short, so that the phase matching condition can be relaxed.

[0044] (2) Since the density of optical waves within the micro-resonatorbecomes very high under resonance condition, signal transmission can beperformed sufficiently even when the coupling length is short.

INDUSTRIAL APPLICABILITY

[0045] As described above, the optical waveguide coupler according tothe present invention is applicable to all fields in which data aretransmitted or processed by use of light guided by an optical fiber oran optical circuit.

1. An optical waveguide coupler including a resonator which is incontact with a plurality of optical waveguides and has a wavelengthdiscrimination function, characterized in that a micro-cylinderresonator is sandwiched between two optical circuit boards each carryinga plurality of optical waveguides such that guided light propagatingthrough each optical waveguide of one optical circuit board and having awavelength satisfying the resonance condition of the resonatorpropagates via the resonator to the corresponding optical waveguide ofthe other optical circuit board and exits from an output end of thecorresponding optical waveguide, and that, by utilization of a sizeeffect attained through size reduction of the resonator, influence ofthe resonator on signal transmission between the optical waveguide andthe resonator is reduced under a nonresonance condition, and a conditionfor phase matching between a guided mode of the optical waveguide andthat of the resonator is relaxed under a resonance condition.
 2. Anoptical waveguide coupler according to claim 1, wherein the size effectof the resonator is such that a size parameter L/λ falls within a rangeof 10 to 100, where L is a peripheral length of the resonator, and λ isthe wavelength of light in vacuum.
 3. An optical waveguide coupleraccording to claim 1 or 2, wherein a micro-resonator is disposed incontact with a multimode optical waveguide in order to provide afunction of a notch filter which reduces to zero intensity of lighttransmitted to a specific mode that satisfies the resonance condition ofthe multimode optical waveguide and the micro-resonator.
 4. (deleted) 5.(deleted)